There are five new tunnels serving Terminal 5 with a combined length of approximately 14km. Tunnelling accounted for some £650 million of the total £4.3 billion
T5 construction budget.

Twin bore tunnels for London Underground’s Piccadilly Line and the Heathrow Express rail link were extended by 1.7km and 1.6km respectively. A 1.3km long road tunnel was built to provide airside vehicle access to T5 from the rest of the Heathrow complex, crossing above both Piccadilly Line and Heathrow Express tunnels and beneath taxiways. A 4.1km, 3m diameter stormwater outfall tunnel provides drainage for the extensive areas of new hardstanding and taxiway. And there are new passenger track transit and baggage transfer tunnels linking all three T5 buildings.

Ground movements arising from tunnelling were considered one of the major risks because of their potential to damage existing infrastructure and cause disruption to rail services or airport operations.

Monitoring and control of ground movement was accordingly a key part of the risk management strategy. While the majority of tunnelling has been carried out using tunnel boring machines (TBMs), innovative developments of a single shell shotcrete lined tunnelling method, called Lasershell, were used for construction of the numerous short, complex, tunnels associated with the main bores.

Risks associated with tunnelling have been reduced by handing construction for all of the tunnels to the same firms. Mott MacDonald carried out design of all tunnels and construction supervision. We also provided specialist advice and interpretation of data from settlement monitors.

A joint venture of Morgan Est and Vinci Grand Projects was tunnelling contractor; Balfour Beatty Rail Projects installed rail and traction power in the two rail tunnel extensions; and Balfour Kilpatrick carried out mechanical and electrical fit-out.

Tunnelling techniques

We carried out construction of the airside road tunnel, Piccadilly Line and Heathrow Express rail extensions and the stormwater outfall tunnel using tunnel boring machines (TBM). Linings for all four consist of precast concrete segments. Those making up the rail and stormwater outfall tunnels use a wedgeblock system, in which lining segments are held in place by the compressive force exerted by the surrounding ground. Lining for the airside road tunnel consists of bolted segments with rubber gaskets waterproofing the joints. This is because sections of the airside road tunnel are at the interface between impermeable London clay and water-bearing terrace gravels lying above.

We constructed the tracked transit tunnel, connecting the concourses of the main T5 terminal building and its two satellites, using the cut-and-cover method.